1. Field of the Invention
The present invention relates to a method of increasing the viscosity of gelatin containing photographic coating solutions.
2. Description of the Prior Art
Recently, there has been a tendency for photographic light-sensitive materials to be made to have a multilayer structure in order to improve the quality thereof. However, in photographic light-sensitive materials having a multi-layer structure, since there is a tendency toward a deterioration of the image sharpness or a reduction of the processing speed, a reduction in the thickness of each layer thereof has been employed to counteract these disadvantages. In order to form thin layers, the thickness of the layers or the coating speed thereof must be maintained. However the thickness of the layers and the coating speed are largely affected by the viscosity of the coating solutions. More specifically, since keeping the thickness and coating speed uniform is impossible where the viscosity of the coating solutions is too low or too high, the coating solutions should have a suitable viscosity. Accordingly, it is necessary to control the viscosity appropriately.
Since it was found by Richard Leach Maddox in 1871 that gelatin could be used as a binder in photographic layers such as a silver halide emulsion layer, a large amount of research has been done on photographic gelatin and the importance of gelatin has been recognized (refer to, for example, Shashin Hattatsushi Shashingijutsukoza Extra Volume, Chapter 22, written by Yasuji Kamata (issued by Kyoritsu Shuppan Co.) or Shashinkagaku pgs. 202-203 3 (issued by Kyoritsu Shuppan Co.)). This is the reason presently why gelatin has been used as one of the most important ingredients of a photographic light-sensitive material as well as silver halide. However, gelatin which has excellent photochemical and colloidchemical properties still has insufficient properties.
For instance, viscosity is one of these properties of gelatin which is insufficient. Namely, if the intention is to form thin layers by increasing the concentration of gelatin in order to increase the viscosity of the coating solutions, drying of the photographic layers begins too rapidly causing difficulties in coating.
In order to solve these technical problems, proposals to use certain kinds of copolymers as thickening agents have been made. For example, a method of increasing the viscosity which comprises adding polyacrylic acid, polymethacrylic acid or copolymers of acrylic or methacrylic acid to gelatin coating solutions (U.S. Pat. Nos. 2,461,023 and 3,655,407 and German Patent Application (OLS) No. 2,427,297) is known. This method, however, is inconvenient because a large amount of the polymers needs to be added if they have a low molecular weight and the viscosity of the coating solutions becomes too high if they have a high molecular weight.